Research On The Contact State Control Method For On-machine Thickness Measurement Using Ultrasonic

Ultrasonic thickness measurement is a feasible measure for measuring the thickness of large thin-walled parts in aerospace field.The ultrasonic sensor is integrated into the NC machining equipment.By controlling the ultrasonic sensor to measure the motion,the thickness of the parts is measured in the machine,and the thickness data is fed back to the machining process to meet the control requirements of the remaining wall thickness of the parts.The contact state of ultrasonic pulse thickness measurement mainly adopts the principle of pulse reflection.The contact state directly affects the coupling state between the probe and the workpiece.The unreliable contact leads to the multi component coupling of the echo signal,which makes the measurement result unbelievable.Therefore,a reliable contact between the sensor and the measured surface is required.In actual measurement,the shape of the measured surface is complex and unknown,which makes it difficult to maintain the contact force between the sensor and the workpiece in the ultrasonic scanning thickness measurement process.If the contact force is too large,it will inevitably scratch the workpiece and even cause the deformation of thin-walled parts.Otherwise,the coupling effect is not good,which affects the quality of ultrasonic echo signal.Therefore,the effective control of contact force is the core problem to ensure the accuracy and stability of ultrasonic measurement.In view of the contact state stability of the ultrasonic thickness measurement,the influence of the friction force on the force of the device is studied.The calculation model of the normal sweep force is established in the process of the machine scanning.The contact force under the free scanning state is simulated and the constant force control strategy under the ultrasonic scanning is put forward.A force-position conversion model based on impedance control is proposed,and the force feedback deviation is converted to the control of the position.A sliding mode variable structure position controller is set up to control the position accurately.Using the least square method to identify the impedance control parameters,and using the Simulink module in Matlab to carry out numerical simulation to verify the effectiveness of the algorithm.The ultrasonic measurement system has improved the stability and thickness measurement accuracy in the machine scanning measurement,and makes some contributions to the problem of measuring the thickness of the large thin wall parts.